Six degrees of Brook Trout: How connected are brook trout populations across streams?

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While angling for brook trout you may have noticed small differences between fish depending on the stream you are angling in even when you are within the same watershed. This forces us to alter the type of flies we use depending on the stream, sometimes even changing as we progress downstream. While most of the fly preferences of trout can be explained by differences in the environment, sometimes it seems that they are simply different fish. Obvious reasons for this are barriers, such as dams, that prevent brook trout in one tributary from breeding with trout from another. A recent paper, however, found that sometimes the reasons are more cryptic than that.

The study by Kazyak et al. (2016) discovered two genetically distinct brook trout populations inhabiting connected tributaries. Brook trout in a stream network in Maryland were examined using DNA analysis and mark-recapture techniques. They found that even though there were no physical barriers separating them, the brook trout rarely moved between tributaries and displayed a very small amount of interbreeding. This resulted in two genetically distinct brook trout populations where one would be expected if only the stream’s physical features were considered. In terms of life-history characteristics, the two populations exhibited similar survival rates and highly variable, but different, growth rates.

The examined stream reaches represent less than 20% of the total, highly connected, network. Each stream in the network is possibly home to a different subpopulation of brook trout, connected through dispersal at such a low rate that the whole network isn’t homogenized into a single population. They term this a “cryptic metapopulation”, which essentially just means an undetected (“cryptic”) connected group of sub-populations of the same species (“metapopulation”).

Saugeen River TributaryBefore embarking on a conservation effort for brook trout, we need to have an understanding of the underlying metapopulation structure. Kazyak et al. (2016) note that having genetically distinct populations spread out over the network can serve as a source of genetic diversity that could increase the sustainability of the metapopulation as a whole. But what this also means is that conservation efforts that attempt to rescue declining subpopulations by increasing the connectivity of stream reaches may be in vain (Kazyak et al. 2016). Due to the naturally occurring limited movement between streams, even in the absence of barriers, it is possible that very few, if any, fish in the other populations will become residents of or spawn in the stream with the declining population. This means that it would take much longer for the stream to be rescued than expected for a homogenous population. Additionally if fish were transplanted to the stream with the declining population, the genetic diversity, hence sustainability, of the meta-population would still be reduced even if the transplant was successful in increasing the abundance of trout in the affected stream reach.

There may be other side effects of removing barriers. While barriers limit dispersal of the brook trout that may be moving between steams, they also prevent the migration of competitors of brook trout. For example, I fish a population of brook trout in Southern Ontario on a tributary of the Saugeen that may only exist because a physical barrier prevents downstream brown trout from establishing. If this population was experiencing a decline, removing the barrier might even speed up the rate of decline by allowing the establishment of a brown trout population.

These observations won’t come as a shock to most fly-anglers. We return to the same hole over and over to target the same fish, Johnny three-spot beside the toppled cedar, so we know there is little movement between streams and can imagine that the populations would be genetically distinct based on the consistent placement of redds throughout the stream. Anglers also experience first-hand failed recovery efforts that didn’t take into account the structure and connectivity of the metapopulation.

Amanda L Caskenette, PhD

Kazyak, D.C., Hilderbrand, R.H., King, T.L. Kellwer, S.R., Chhatre, V.E. 2016. Hiding in plain sight: a case for cryptic metapopulations in brook trout (Salvenius fontinalis). PLoS ONE 11(1): e0146295. Doi:10.1371/journal.pone.014629

This paper is open access and can be downloaded here.

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